Electrical conductor lines when installed in buildings or underground are usually surrounded and protected by corresponding lengths of conduit. Fiber optic lines used for transmission of communication signals likewise are surrounded by conduit and run in a similar manner.
Typically these lines and the surrounding conduit run for extended distances of thousands of feet under streets through systems of interconnected tunnels with access manholes. Installation of the lines in the subterranean conduit tubing has been a very difficult matter requiring substantial time and effort, particularly in light of the cramped conditions present in the field.
Typical past practice has involved excavating tunnel areas, installing the desired network of conduit tubing and then installing the conductor or transmission lines in the conduit. This latter has been accomplished by running a strong and heavy pull rope or cable in the conduit, attaching the pull rope to the conductor line, and then pulling the pull rope back through the length of conduit and the conductor line along with it. In order to accomplish this operation it is first necessary, however, to run the pull rope through the conduit. This has been done in the past typically by using means and apparatus such as shown in U.S. Pat. Nos. 3,006,607; 3,120,947; and 3,246,878 (all issued to assignee herein) wherein a lighter weight pull line is run through the conduit by fluid pressure pushing or pulling through the conduit a line package of pull line. The pull line is then connected to the heavier pull rope and pulled back through the conduit, pulling the pull rope back through with it. The line package of pull line usually has been provided as a single cylindrical shaped package up to about six inches in length with an outside diameter less than the inside diameter of the conduit. Such a package has contained up to about 500-600 feet of pull line having about a 27 pound tensile strength.
These figures represent what has been considered to be a maximum length and size for pull line, since use of a package comprising greater quantities of heavier line would yield a package of a size that could not readily be passed through the conduit, particularly the curves and bends thereof. Although a greater quantity of lighter weight line could yield a package of smaller size that could be accommodated by the conduit, the line would not be strong enough to pull back the heavier pull line through the conduit--particularly conduit extending the longer distances of over six hundred feet.
Being restricted to use of these shorter distances meant that repeated installation operations were required at multiple locations and that numerous line splicings were required in the field. Such numerous splicings of conductor lines were undesirable since in addition to the additional expenditure of manpower and effort required to effect the splicing operations, the presence of the splice typically adversely affected the electrical conductivity of the line and required the installation of additional electrical amplification units.
Furthermore, the problems created by splicing have become even more acute recently in connection with the installation and splicing of fiber optic lines for communication signal transmission. Splicing of fiber optic lines is a technically difficult procedure in the first place requiring skilled techniques to properly accomplish. Furthermore, fiber optic line splices tend to substantially adversely affect the transmitted signal and typically require additional electronic means to correct adverse effects caused by the splices.
Accordingly, it has become quite desirable to install longer continuous lengths of conductor lines, and particularly fiber optic lines, in conduit.